Porous ceramic articles find utility in a variety of applications where resistance to high temperature or chemical attack is important. U.S. patent application Ser. No. 11/394,594 filed Mar. 30, 2006 entitled “Reactive Binders For Porous Wall-Flow Filters,” now U.S. Pat. No. 7,575,618, and U.S. patent application Ser. No. 11/432,038 filed May 10, 2006 entitled “High Porosity Cordierite Composition,” now U.S. Pat. No. 7,648,548, are each hereby incorporated by reference in their entirety.
The processing of ceramic materials usually produces a finished article with a porosity of at least above five percent. Higher porosity is desirable in certain applications, such as exhaust gas converters for internal combustion engines, combustion elements for boilers, reformers of liquid fuel or gaseous fuel, and purification systems for water or sewage. Many high porosity ceramic articles used in these applications have a honeycomb structure to ensure large surface area.
High porosity ceramic articles have a total porosity above about twenty percent. For example, commercial high porosity filters may have porosities from about 40-65%, for example. High porosity can be achieved by mixing a pore-forming agent with ceramic-forming inorganic batch materials, a binder and processing aids to form a plasticized batch mixture, forming the plasticized mixture into a green body, and firing the green body to form the ceramic article. The pore-forming agent is generally used to increase the number of pores and to control the size and volume of pores in the porous ceramic article. In higher porosity articles, the amount of pore former needed increases. As a result, cracking becomes more problematic during both the drying and firing steps. Accordingly, forming ceramic articles with high porosities may result in low yields. Dry yields are the percentage of green bodies that survive drying without significant cracking or deformation. Likewise, firing yields are the percentage of fired bodies that survive firing without significant cracking or deformation. Obviously, high dry and firing yields reduce manufacturing cost and improve efficiency and are, therefore, sought after.
Pore forming agents include carbon-containing compounds, such as graphite, starch, and various organic polymers and resins that will volatilize during firing of the green body. Pore forming agents also include foaming agents and inorganic, hollow particles such as silica micro-balloons. The amount of pore forming agent depends on the desired porosity of the finished ceramic article. For example, a honeycomb ceramic article with approximately thirty percent porosity may be produced from a green body comprising 100 parts by weight of a ceramic-forming powder, up to about 30 parts by weight of an organic pore forming agent, and a sufficient amount of a vehicle, such as water. During firing, the organic pore forming agent volatilizes leaving vacant spaces, that is, pores, in the finished ceramic article. Pores greatly increase the surface area of the article and can even increase thermal shock resistance of the article. In particulate wall-flow filter applications, the pores provide the interconnected flow path for filtration of particulates from the flow stream.
Starch and graphite are used as pore forming agents. Drying of green body articles including these pore formers required a low level of heating. Starch softens at fairly low temperatures, around 80° C., so that a green body having a high fraction of starch can slump during drying, thereby reducing drying yield. One proposed solution is to crosslink the starch before adding it to the mixture. This increases the softening point of the starch and reduces slumping. However, this does nothing to increase the strength of the green body or reduce cracking. Graphite does not soften during drying. However, when microwave radiation is used in the drying processes, the graphite may heat up very quickly, leading to cracking. Accordingly, each pore forming has its drawbacks.
In some applications, higher porosity ceramic articles are desired. For example, high porosity ceramic filters in the exhaust system of a diesel truck can both increase the life of filter and decrease pressure drop through the filter, thereby increasing fuel efficiency of the engine. Manufacturers would like to produce very high porosity ceramic articles, that is, porosities of above fifty-five percent, preferably over sixty percent, and more preferably over sixty-five percent. However, achieving such high porosities is a challenge while maintaining acceptable drying and firing yields. Higher porosity generally means more pore-forming agents, possible thinning of the ceramic walls in the green body with a decrease in strength, and generally lower drying yields. Prior art solutions for increasing drying yield in high porosity ceramic articles include extremely long drying cycles and drying in high humidity conditions. Both solutions interfere with standard manufacturing processes and significantly decrease manufacturing speed.
As such, a need exists for a high porosity ceramic article, and a method of manufacturing such a ceramic article so that cracking of the green body is reduced. Preferably, the article and method will be compatible with existing manufacturing processes.